A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus are conventionally used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures.
It is desirable to reduce the size of features in a lithographic pattern because this allows for a greater density of features on a given substrate area. In photolithography, the increased resolution may be achieved by using light of shorter wavelength. However, there are problems associated with such reductions. Current systems are starting to adopt optical sources with wavelengths in the 193 nm regime but even at this level, diffraction limitations become a barrier. At lower wavelengths, the transparency of materials is very poor. Optical lithography machines capable of enhanced resolutions require complex optics and rare materials and are consequently very expensive.
An alternative for printing sub-100 nm features, known as imprint lithography, comprises transferring a pattern to a substrate by imprinting a pattern into an imprintable medium using a physical mould or template. The imprintable medium may be the substrate or a material coated on to a surface of the substrate. The imprintable medium may be functional or may be used as a “mask” to transfer a pattern to an underlying surface. The imprintable medium may, for instance, be provided as a resist deposited on a substrate, such as a semiconductor material, into which the pattern defined by the template is to be transferred. Imprint lithography is thus essentially a moulding process on a micrometer or nanometer scale in which the topography of a template defines the pattern created on a substrate. Patterns may be layered as with optical lithography processes so that, in principle, imprint lithography could be used for such applications as IC manufacture.
The resolution of imprint lithography is limited only by the resolution of the template fabrication process. For instance, imprint lithography may be used to produce features in the sub-50 nm range with significantly improved resolution and line edge roughness compared to that achievable with conventional optical lithography processes. In addition, imprint processes do not require expensive optics, advanced illumination sources or specialized resist materials typically required by optical lithography processes.
Imprint lithography may be used to manufacture multiple layer structures, such as integrated circuits. In order to manufacture such multiple layer structures, each layer is typically independently imprinted. For example, a first layer is imprinted on a substrate, following which the imprinted material is treated such that the imprinted pattern becomes fixed. A second layer is then imprinted on the first layer and treated, followed by third and fourth layers, etc. In some integrated circuit designs up to thirty layers may be used. Manufacture of multiple layer structures is commonplace using optical lithography apparatus. However, it has so far proved difficult to manufacture such structures using imprint lithography, due to difficulties in aligning the patterns of successive layers such that they correspond correctly. Alignment between successive layers should be achieved with high accuracy otherwise device components, which are constructed by several layers, will not contact one another or do so improperly with the result that the device components may not function as intended.